Electronic devices, such as cellular telephones, cameras, and computers, commonly use image sensors to capture images. A typical CMOS (complementary metal-oxide-semiconductor) imager circuit includes a focal plane array of pixels, and each pixel includes a photo-sensor, such as a photogate, or photodiode, for accumulating photo-generated charge in a portion of the substrate. Each pixel has a charge storage region, formed in the substrate. The charge storage region may be constructed as a floating diffusion region. In some imager circuits, each pixel may include at least one electronic device, such as a transistor, for transferring charge from the photo-sensor to the storage region, a second device, also typically a transistor, for resetting the charge storage region to a predetermined bias level prior to charge transference, and a third device for amplification or buffering of the signal prior to connection to a readout line.
In a CMOS imager with charge integrating pixels or pixels that operate in integrating mode, the active elements of a pixel perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the storage region; (4) selection of a pixel for readout; (5) output and amplification of a signal, and (6) resetting the storage region to a known state representing a reset level
Many conventional high-performance image sensors are described by one or more specifications, such as high resolution, high dynamic range, high speed, low noise, low dark current, no image lag, charge storage capacity, output voltage swing, etc. While some of the specifications are interrelated, some are trade-offs due to the physical properties and the design of the image sensor. For example, conventional image sensors may utilize a source follower transistor to buffer the charge generated voltage at the storage region to a pixel output voltage. The source follower transistor, however, adds noise to the signal without increasing the gain of the signal. Other considerations may include the pixel size of the image sensor, as smaller image sensors are preferred for various electronic devices, and/or the cost to fabricate the image sensor.